Mobile communication systems are continually evolving into high-speed, high-quality wireless packet data communication systems for data service and multimedia service provision beyond the traditional voice-oriented service, which was often considered relatively restrictive. Presently, a 4th-Generation (4G) communication system is in process of standardization. That is, a 3rd Generation Partnership Project (3GPP)/3GPP2 is evolving into a Long Term Evolution-Advance (LTE-A) following LTE, and an Institute of Electrical and Electronics Engineers (IEEE) 802.16 Wireless Metropolitan Area Network (WMAN) camp is evolving into an IEEE 802.16m advanced mobile World Interoperability for Microwave Access (WiMAX) following an IEEE 802.16e WiMAX, and the like while they support a high-quality wireless packet data transmission service.
Accordingly, in the future, it is expected that many wireless mobile communication systems will be developed for the purpose of increased system capacity that provide high-speed, high-quality wireless packet data transmission. A Distributed Antenna System (DAS) has been devised as one potential technique of providing this.
The DAS was initially devised with the concept of a relay for shadow area service. Accordingly, the DAS includes a form similar to that of an initial relay wired or wirelessly receiving a signal of a Base Station (BS) located in the center of a cell and repeatedly transmitting the same signal. For the purpose of system performance improvement, a Coordinated Multipoint Transmission (hereinafter, referred to as ‘CoMP’) technology for transmitting different signals to respective antenna nodes and enhancing system performance has been developed. In many cases, the CoMP technology extends beyond the concept of the traditional relay simply repeatedly transmitting the same signal. Afterwards, the CoMP technology has been evolving into a technology that considers independent reuse of frequencies for respective antenna nodes while being differentiated from the initial relay technology.
FIGS. 1A and 1B illustrate the concept of a macro cell and a DAS according to the conventional art.
As in FIG. 1A, in an environment in which an initial relay is used, a BS having a high output capacity exists in the center of a macro cell, and a small output relay (not shown) exists in a shadow area. This particular macro cell is presumed to have a radius (R) of about 1 kilometer (km). A terminal communicates with the BS within the macro cell of the 1 km radius.
As in FIG. 1B, in the DAS, several antenna nodes (i.e., 7 antenna nodes) are distributed and installed in a macro cell service area having a radius of 1 km, and each antenna node may simultaneously transmit a different signal. In this aspect, the DAS may be thought as a wide Multiple Input Multiple Output (MIMO) system, and has a difference with the MIMO system that each antenna node is spatially separated from one another. Due to this, the MIMO technology may also be applied to the DAS technology.
Further, the DAS technology may use distributed antenna nodes as small cells within a macro cell service area in an existing cellular network, so the DAS technology may use the antenna nodes as additional frequency resources. Through this, system capacity may be increased. Also, because each small cell may have a small service area, a conventional macro cell having a small output amplifier is made usable and thus, a power loss is not only less but also interference in adjacent cells may be reduced. Owing to this characteristic, the DAS technology is increasing as the core of 4G or post-4G technology, such that intensive research and standardization efforts are in progress.
On the other hand, a DAS technology is a form of connection between a centralized controller and distributed antenna nodes. A significant feature of distinguishing cells composed of the distributed antenna nodes from existing independent cellular cells is that the DAS technology couples the distributed antenna nodes to one centralized controller. Accordingly, in a case where a terminal moves from one particular cell to another cell of the independent cellular cells, because each cell is independently controlled, a separate signal process associated with terminal movement between respective cells may be needed. Unlike the DAS technology, in a case where a terminal moves from a particular antenna node to another antenna node, antenna nodes that have been already connected and controlled by one centralized controller, there exists a possibility that processing in one centralized controller may occur without a separate signal exchange procedure between the antenna nodes.
The DAS technology, however, does not consider the mobility of a terminal, so it is not suitable for supporting a high-speed mobile terminals. That is, in a case where there are many terminals moving at a high speed, the terminals may pass multiple small cells or antenna nodes in a short time. Accordingly, to recognize the small cells or antenna nodes that the terminal passes, the DAS technology often requires a separate signal exchange or mutual authentication procedure between the terminal and the small cells or antenna nodes. Accordingly, it has been recognized that due to overhead, the DAS technology is not suitable for servicing mobile terminals, and more particularly, high-speed mobile terminals.